One of the coolest and hardest things I've ever done in life, completely free for first-timers (include food, stay, and meditation lessons for 10 days): https://www.dhamma.org/en-US/locations/directory
For me, waking up at four every morning, eating all vegan food, no dinner (just fruits), and living in a completely secluded area without contact to the outside world (the only places we can go are our dorm room, the meditation hall, the food court, the courtyard, and the public bathroom. It feels like a prison with pretty nice facility). And we can't talk to anyone other than asking the teacher and the logistics staff questions, nor can we have eye contact or physical contact with anyone. All of these are actually not bad at all.
What's one of the toughest is battling with your physical self because there were a few days that you are supposed to sit for one hour straight meditating (Vipassana meditation is simply observing your entire body objectively, knowing what each part of your body feels like, hot-cold pain sweat vibration, etc. Then you have to realize the rising and falling nature of these feelings and sensations and train your mind to keep an equanimous heart without hatred or greed toward one sensation over another), because every time I sit for longer than 30 min my legs start to puff up with all the blood (so puffy that when I touch it, I almost can't feel my bones)and then I experience the sensation of cramped muscle every second and then my kneecaps hurt like someone is twisting them. Then my hip/back hurt because I have herniated lumbar disk and inflammation in my pelvis. Every time I sit to meditate, I can't move for about 5 mins bc I have to gradually open my eyes, run my fingers, slowly move my hands, slowly move my legs with my hands, slowly put my legs in a different position, one time it took me 10 mins to get out of my seat and walk about 20 ft away. With that, we are supposed to meditate 12 hours a day.
But even all that isn't the toughest, the toughest is battling with your thoughts and your heart. I realized that when I am pretty much alone, have no source of contact with the outside world; I start to imagine things. One thought might be heaven, and the next thought leads to hell. My mind starts to create false imaginations of what's happening out there, a lot of times its completely unreasonable stress and worries (e.g., I was imagining one of my best friends dying, or my grandma gets super sick and can't see me because I am here). Random thoughts cannot stop popping out because it's our nature, what we can do is ignore them and focus on what we are supposed to be focusing on (sensing the sensations of our body). But even with that, sometimes you just go in a spiral of negative emotions that you don't have enough will power to pull yourself outside of that. So the worst pain is the pain from the heart, which is what we create for ourselves. We are so indoctrinated to automatically feel a certain way (hatred or greed, liking or disliking something) when anything happens to us that we are not even aware that we've been pulled away by our emotions, so one of the most significant insights is becoming the master of myself and my emotions whenever I realized that they are controlling me